Disclosed in embodiments herein are methods and systems for generation of a color watermarked image having a watermark embedded in different color channels of the image using halftone structures that meet a uniform-rosette condition, also referred to as minimum-rosette halftone screens. The color watermark can be retrieved, or viewed, using an overlaid reference key constructed with halftone structures meeting similar uniform-rosette conditions.
With the advent of inexpensive digital color printers, methods and systems of color digital halftoning have become increasingly important in the reproduction of printed or displayed images possessing continuous color tones. It is well understood that most digital color printers operate in a binary mode, i.e., for each color separation, a corresponding color spot is either printed or not printed at a specified image location or pixel. Digital color halftoning controls the printing of color spots, where the spatial averaging of the printed color spots by either a human visual system or a viewing instrument, provides the illusion of the required continuous color tones.
The most common halftone technique is screening, which compares the required continuous color tone level of each pixel for each color separation with one or more predetermined threshold levels. The predetermined threshold levels are typically defined for a rectangular cell that is tiled to fill the plane of an image, thereby forming a halftone screen of threshold values. At a given pixel if the required color tone level is darker than the threshold halftone level, a color spot is printed at that specified pixel. Otherwise the color spot is not printed. The output of the screening process is a binary pattern of multiple small “dots”, which are regularly spaced as determined by the size, shape, and tiling of the halftone cell. Conventional screening outputs can be considered as two-dimensional repeated patterns, possessing two fundamental spatial frequencies, which are completely defined by the geometry of the halftone screens.
Halftoning images can also provide significant desirable visual effects. If two similar cluster halftone image patterns are superimposed on each other, the output can appear significantly different depending on the relative positions of the dot patterns as defined by their phase shifts.
Prior patents, such as U.S. Pat. No. 6,252,931 for “Digital watermarking using phase-shift stoclustic screens,” by S. Wang, hereby incorporated herein by reference in its entirety, describe a method to embed correlation-based phase-shift digital watermarks, also referred to a correlation marks, into halftone screens. By overlaying a transparency on the prints generated by the special halftone screen, for example as a public key, an invisible watermark embedded in the image can be retrieved.
For example, the two checkerboard patterns, 100 depicted in FIG. 1A and 110 depicted in FIG. 1B, are essentially the same, except that the pattern 110 in FIG. 1B is a shifted version of the pattern 110 in FIG. 1A with an exactly “one-box width” shift. If the two patterns 100 and 110 are superimposed on each other with a perfect alignment, the result would be a completely black image as depicted by 120 in FIG. 1D. On the other hand, overlapping pattern 100 of FIG. 1A with itself, which can be considered another version of 100 with a zero-shift, gives an identical pattern to the original pattern 100, as depicted by 130 in FIG. 1C.
Referring now to FIGS. 2A-2C, a halftone pattern is shown at 200 having only it's central portion, shown as region 210, shifted in this manner. When the reference, or “public key”, represented by the halftone pattern 240 of FIG. 2B is overlaid on top of the pattern 200, the result is clearly visible as a black central region shown at 250 in FIG. 2C. The example depicted in FIGS. 2A-2C is a simple demonstration for the phase-shift digital watermark technique. The shifted central part 210 in the picture may be considered a watermark, which is retrieved as a black circular digital watermark, or correlation mark, 250 in the overlay shown in FIG. 2C. The shift required for an optimal retrieval is equal to a half period of the halftone structure, or π, in a general mathematic term. The problem with a simple “insertion”, however, is that the boundaries between the shifted portion 210 and the balance of the image can be quite visible as a seam 220 shown in FIG. 2A.
Several different approaches may be taken to extend the method for embedding phase-shift correlation-marks for the black/white images, describe above, to color halftoning.
First, it can be achieved by conducting dot-on-dot screening, i.e., applying the embedding to all color channels exactly the same way as for the black/white case. However, it is well known in color printing world that a dot-on-dot color halftoning configuration suffers from mis-registration between different color channels. Other drawbacks of the dot-on-dot configuration includes reduced color gamut compared with other configurations. Therefore, the dot-on-dot approach for correlation-marks is somewhat limited and many high-quality color printers use separate halftone screens with different angles and/or frequencies for different channels.
The second possible approach for embedding correlation-marks into color images is embedding the watermarks into different channels independently based on the geometries of halftone screens being used for each color. However, different keys, or overlay transparencies, are required to retrieve the watermarks for each of the individual colors. Also, this approach provides satisfactory results for instances when correlation-marks are embedded into areas where one of the primary colors is dominating and the transparency for watermark retrieval can be selected correspondingly. For areas with mixed colors, the contrast of the retrieved watermarks will be severely reduced if the images are halftoned by using different rotated screens. Thus, this second approach also has limited applications.
Furthermore, the superposition of halftone screens for color printing can create interference patterns, known as moiré, which can be seen in the image, thus detracting from the visual appearance of the halftoned image. Significant efforts have been undertaken to reduce the undesirable effects of moiré in color halftoning.
An aspect of the disclosed system and method provided extends use and formation of correlation marks by enabling mulit-color watermarked images to be formed by embedding a phase shifted (correlation-based) digital watermark into the color halftone used for each color separation, or channel, by halftoning the color input image with different one of a plurality of three-dimensional threshold arrays where at least one input thereto is a phase shift value using a different halftone structure for each channel. Each halftone structure is generated meeting uniform rosette conditions, also known as minimum rosette conditions. The color separations are combined to form an output color image having the digital watermark embedded therein but not visible to the unaided eye. A single common key, in the form of a transparency having a matching periodic structure, can be used to retrieve the watermark from the output color image producing a full color image of the output image having the watermark image visible therein.
Disclosed in embodiments herein is a method for digitally reproducing a moiré-free color halftone image having an embedded correlation-based digital watermark using an enhanced halftone screen set consisting of a halftone screen for each of N colorants forming N color separations (where N≧3), including providing a color input image to be watermarked; providing a watermark image to be embedded in the N-color image; generating N different uniform rosette halftone screen configurations each meeting uniform rosette halftone screen conditions; generating N three-dimensional threshold arrays each having a phase shift value as an input; and halftoning the color input image by halftoning N different color separations using a different one of the halftone screen configurations and three-dimensional threshold arrays for each color separation to produce a moiré-free color output image having the watermark image embedded therein.
Also disclosed in embodiments herein is a system for digitally reproducing a moiré-free color halftone image having an embedded correlation-based digital watermark using an enhanced halftone screen set consisting of a halftone screen for each of N colorants forming N color separations (where N≧3), including an input image source providing a color input image; image memory for storing the input image to be watermarked; watermark memory for storing the watermark image to be embedded in the color input image; and an image processor for generating N (where N≧3) different uniform rosette halftone screen configurations each meeting uniform rosette halftone screen conditions, generating N three-dimensional threshold arrays each having a phase shift value as an input, and halftoning the color input image by halftoning N different color separations using a different one of the halftone screen configurations and three-dimensional threshold arrays for each color separation to produce a moiré-free color output image having the watermark image embedded therein. Wherein, as disclosed herein, the uniform rosette halftone screen conditions include: defining rosette fundamental frequency vectors VR1, VR2 that satisfy a length and sum requirement to meet visual acceptability standards according to |VR1|>fmin, |VR21|>fmin, and |VR1±VR2|>fmin; defining N halftone screens for color separation i=1, N, respectively (where N≧3), possessing first and second frequency vectors (Vi1, Vi2), where no two screens possess identical fundamental frequency vector pairs; and selecting fundamental frequency vectors for the N halftone screens according to (Vi1, Vi2)=(mi1VR1+mi2VR2, ni1VR1+ni2VR2) for integer m's and n's, where for each color separation i, at least one fundamental frequency vector or its conjugate must also satisfy the following inequality: |Vik|>max [|VR1, |VR2|, min[|VR1+VR2|, |VR1−VR2|]], k=1 or 2.
The various embodiments described herein are not intended to limit the invention to those embodiments described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.